Run tests in venv

Previously we had wrongly assumed that python yaml package
will be available in the images and the run-tests.sh script
ran a set of things which included a script that used
the pyyaml package. With the recent bindep changes, this
assumption broke and the gate-devstack-gate-unittests
failed. So we need to install the pyyaml package and then run
the tests in this venv for run-tests.sh to work.

Change-Id: I541c8add8041348a259b3b789d24cf24ba122ce0

test-requirements.txt

@@ -0,0 +1 @@
+PyYAML>=3.1.0

tox.ini

@@ -7,6 +7,11 @@ skipsdist = True
 install_command = pip install -U {opts} {packages}
 setenv = VIRTUAL_ENV={envdir}
 
+[testenv:run-tests]
+deps = -r{toxinidir}/test-requirements.txt
+commands =
+  bash -c "./run-tests.sh"
+
 [testenv:bashate]
 whitelist_externals =
   bash